Low frequency radio-frequency identification (RFID) devices, such as automotive immobilizer devices and the like, typically conform to ISO 11784, 11785 and 14223 standards defined by the International Organization for Standardization (ISO) for radio-frequency identification of animals. These standards define two protocols for communicating between a basestation transceiver device (also known as a ‘reader’) and a transponder device (also known as a ‘tag’).
The first of these protocols is a full-duplex communication protocol utilizing load-modulation with amplitude shift keying (ASK) for transponder-to-basestation communication. Basestation-to-transponder communication is achieved by the basestation transceiver device generating an AC magnetic field, and modulating the magnetic field in the amplitude domain to convey information to the transponder device. Transponder-to-basestation communication is achieved by the basestation transceiver device generating a constant AC magnetic field and the transponder device modulating the load it presents to its inductive antenna. Due to the inductive proximity coupling between the basestation transceiver device and the transponder device, modulating the load presented to the inductive antenna of the transponder effects the magnetic field. Load-modulation is robust against noise in the communication channel due to good signal amplitude and low system bandwidth requirements. However, the load-modulation effect on the magnetic field is small relative to the constant carrier generated by the basestation transceiver device, requiring a very high dynamic range for the basestation transceiver device in order to detect the load modulation of the transponder device, limiting the achievable communication distance.
The second protocol is a half-duplex (charge and talk) communication protocol utilizing frequency shift keying (FSK) for transponder-to-basestation communication when the magnetic field generated by the basestation transceiver device is turned off. Since the signal from the transponder is not required to be detected over the much larger signal generated by the basestation transceiver device, the dynamic range requirements for the receiver of the basestation transponder device are significantly lower than for a full-duplex implementation, enabling lower energy transmission by the transponder device. However, the bandwidth requirements for FSK modulation are higher than for ASK modulation, and the charge and talk FSK system is less robust against noise in the communication channel than for the load-modulation system.